The invention concerns a method of coating materials by plasma powder build-up welding.
The operation of coating iron-based materials with materials in powder form on a nickel, cobalt and/or copper basis has long been implemented in industry. When applying materials which oxidize easily such as for example aluminum, titanium or alloys thereof, the coating operation is very severely disturbed and possibly rendered entirely impossible to implement, due to the oxides which are formed at the surface.
Even attempts to conduct those operations under a protective gas failed. The only possible way of implementing such a coating operation is the very expensive and complicated procedure of operating under vacuum, which has never resulted in use on a large technical scale.
In accordance with German Patent Publication 05 196 26 941, to attain the object of permitting the coating of materials which oxidize easily--in particular light metals or alloys--in a simple and inexpensive manner, the inventor proposed the application to an aluminum alloy of a coating with an additive material in powder form by means of powder build-up welding with a plasma transferred arc (referred to as PTA), by the use of an alternating current or a direct current with a superimposed alternating current; by virtue of that procedure, the oxide skin which is formed due to the oxygen in the air or the residual oxygen in the protective gas, at the surface of the base material or of the additive material in powder form, is destroyed or broken up. That makes it possible to carry out coating operations even on or with those easily oxidizable materials, and to achieve a good weld quality. An alternating current in a frequency range of between 10 and 100 Hz is preferred in that procedure. When operating with a direct current as the energy carrier, then--in order to achieve the same properties in terms of welding procedure--it is necessary to associate with the direct current an alternating current at a frequency of between about 10 and 1000 Hz or in the frequency range of between about 1 and 200 KHz.
It is also proposed that the endeavor be made to achieve a high degree of mixing with the base material and it is further proposed that in accordance with the invention the flow speed of the issuing gases be so selected that the slags or oxides which are formed by virtue of the residual oxygen are torn by the gas flow and deposited at the edge of the molten bath.
Also in accordance with the previous invention is a plasma torch with a gas duct of a burner nozzle, the gas duct being arranged upstream of a nozzle opening of narrower cross-section, and with a bar-type cathode which is rounded at its end and in particular is of a spherical configuration, for operation with a superimposed alternating current, for transfer of the plasma; that cathode end forms the base point of the plasma arc which is produced. In other respects attention is directed to the content of the reference and the particular configuration of the cathode; that spherical cathode end forms the base point of the plasma arc which is formed.
A plasma torch for a transferred arc of the kind set forth in the opening part of this specification, with a round bar electrode--disposed upstream of a chamber arranged upstream of the nozzle opening--, with the electrode having a free end which is bevelled therearound, are admittedly known from EP-A-0 452 494. There is however no reference therein to the structure discussed.
In the plasma torch of the previous invention the bulging shaping of the cathode can be so arranged that it touches a plane determined by the end face of an anode surrounding the cathode, or projects outwardly beyond that plane by a short distance, about at most between 3 and 4 mm.
A preferred arrangement has a cathode which is set back into the gas duct of the torch nozzle and whose spherical configuration is then at a short spacing--also of at most 4 mm--relative to that plane and at a gap spacing relative to a shoulder step which forms a transition between the gas duct and the following nozzle opening of narrower cross-section. That ring-like shoulder step is adjoined by a cylindrical portion of the gas duct as a receiving means for the shaped configuration of the cathode and with same and the shoulder step defines the passage for the gas to pass therethrough.
It has proven to be desirable for the cathode which is supplied with an alternating current or with a direct current with superimposed alternating current to be equipped with water cooling. The cathode can be for example in the form of a water-cooled support body with interchangeably fitted cathode portion--of material with a high melting point and with a low level of thermal conductivity--for the shaped configuration or for the base point of the arc; the support body is advantageously formed from copper or a copper alloy.